1. Field of the Invention
The present invention relates to control methods used in resonant converters. More particularly, the present invention relates to individual converter control and paralleled resonant converters with active current-sharing control.
2. Description of the Prior Art
Generally, a resonant converter uses a resonant-tank circuit to shape switch voltages or current waveforms, or both, to minimize switching losses and to allow high-frequency operations without compromising conversion efficiency. Resonant converters are therefore extensively used in state-of-the-art power supplies that offer the highest power densities and efficiencies. Resonant converters are also preferred as power supply circuits for medical applications because of their superior electromagnetic interference (EMI) performance relative to their “hard”-switched counterparts. As resonant converters absorb component parasitics into their resonant-tank circuit (e.g., output capacitance of semiconductor switches, leakage or magnetizing inductance of transformers, or any combination thereof), resonant converters are used in applications where the parasitics are pronounced, such as high-voltage power supply circuits and contactless or inductive power transfer applications.
FIGS. 1(a) and 1(b) show two typical topologies for isolated resonant power converters. FIG. 1(a) shows a half-bridge topology, in which the resonant tank circuit includes inductor LR and capacitors CR1 and CR2. FIG. 1(b) shows a full-bridge topology, in which the resonant tank circuit includes inductor LR and capacitor CR. Because in both resonant tank circuits the resonant inductor is connected in series with the resonant capacitor or capacitors, the circuits of FIGS. 1(a) and (b) are series-resonant converters. If the magnetizing inductance of transformer TR is relatively small (e.g., only several times the inductance of resonant inductor LR), the converters operate as LLC series-resonant converters. To maximize conversion efficiency in applications with relatively low output voltages (e.g., 60 volts or less), secondary-side diode rectifiers are usually provided by synchronous rectifiers. In FIGS. 1(a) and 1(b), the synchronous rectifiers are implemented by low on-resistance metal-oxide-semiconductor field effect transistors (“MOSFETs”). Using MOSFETs as synchronous rectifiers also makes the resulting converter bidirectional because controllable switches on both the primary side and the secondary side of the transformer allow power to flow in both directions. The secondary side of an isolated resonant converter can be implemented by a single secondary winding and a full-wave rectifier, rather than the center-tapped secondary winding shown in FIGS. 1(a) and (b).
Generally, a resonant converter operates with variable switching-frequency control. When operating above the resonant frequency, a resonant converter operates with zero voltage-switching (ZVS) in the primary switches. Alternatively, when operating below the resonant frequency, a resonant converter operates with zero current-switching (ZCS). The article “Resonant, Quasi-Resonant, Multi-Resonant, and Soft-Switching Techniques—Merits and Limitations,” by M. M. Jovanović, published in the International Journal of Electronics, Vol. 77, no. 5, pp. 537-554, November 1994, discloses in detail resonant converter topologies and control.
FIG. 1(c) shows waveforms of switch-control signals for ZVS operation in series-resonant converters, such as those shown in FIGS. 1(a) and (b). As shown in FIG. 1(c), each switch operating with a 50% duty ratio, with the primary switches of the same leg (i.e., switches SP1 and SP2, or Sp3 and SP4) operate in a substantially complementary (i.e., non-overlapping) fashion to avoid cross-conduction. A feedback control loop providing output regulation determines the frequency of primary switch operations. On the secondary side, the synchronous rectifier switches are synchronized to switch at the zero crossings of the resonant current. Specifically, synchronous rectifier SS1 is synchronized to turn on at the moment resonant current iP changes from negative to positive and turn off at the moment resonant current iP changes from positive to negative, while synchronous rectifier SS2 is synchronized to turn on at the moment resonant current iP changes from positive to negative and turn off at the moment resonant current iP changes from negative to positive, as illustrated in FIG. 1(c). To achieve ZVS in a practical implementation, the duty ratio of each primary switch is set to a value that is slightly less than 50% by introducing a short delay between the turning-off and the turning-on of its complementary switch in the same leg. During this dead time (i.e., when neither complementary switch is closed), the current is commutated from the switch that is being turned off to the anti-parallel diode in the complementary switch, so as to create a condition for the complementary switch's subsequent ZVS turning on. Typically, each secondary-side synchronous rectifier of the resonant converter is also operated with a duty ratio that is slightly less than 50%.
Variable switching-frequency control is generally seen as a drawback of a resonant converter, especially in an application with a wide input voltage range or a wide output voltage range. Specifically, as the input or output voltage range increases, the control frequency range also increases, so that driving and magnetic component losses also increase, thereby reducing conversion efficiency. Furthermore, in many applications, the converter is restricted to operate within a relatively limited frequency range to avoid interfering with other parts of the system. While a resonant converter can operate with a constant frequency (i.e., in a “clamp-mode” operation), such an operation is not desirable because the increased circulating energy in the resonant tank circuit significantly degrades conversion efficiency.
In a series-resonant converter, such as that shown in FIG. 1(a) or FIG. 1(b), the output voltage ripple is determined by the AC-component of the secondary-side rectified resonant current flowing through output capacitor CO. Such a converter is more suited for a relatively low output current operation. To achieve a low output-voltage ripple in a high-current application, a lager output capacitor is required. The output capacitor is typically implemented by parallel electrolytic capacitors or ceramic capacitors. For converter applications that require a relatively long life time (e.g., an automotive on-board charger, an automotive DC/DC converter, a solar inverter, or an LED driver), it is not desirable to use electrolytic capacitors.
The output capacitor can be significantly reduced by interleaving—i.e., by operating multiple converters in parallel and providing a phase shift between the gate-drive signals. Interleaving substantially cancels the current ripples at the input and output. By interleaving, a resonant converter with a relatively small C-type output capacitor (e.g., the resonant converters of FIGS. 1(a) and (b)) can be used in a high output current application. However, interleaving variable frequency-controlled resonant converters requires that the switching frequencies be synchronized (i.e., the parallel converters operate at the same variable frequency). However, because of the inevitable mismatch between the resonant tank components, the interleaved resonant converters would not equally share a load current (or power) even with identical input and output voltages and switching frequency. Therefore, when mismatched components are present, additional control means (i.e., a control mechanism that is independent of the switching-frequency control) is required to achieve an acceptable current sharing among the interleaved resonant converters.
U.S. Patent Application Publication 2012/0153730, entitled “Interleaved LLC Converter Employing Active Balancing,” discloses a control method for interleaving LLC converters with active current balancing. Under that control method, the input terminals of each interleaved converter is connected to a separate DC power source and the power source voltages (i.e., the input voltages of the LLC converters) are controlled to achieve and to maintain current balance between the interleaved converters. Such a method cannot be applied in an application where the input voltages of the LLC converters are not controllable or if only one DC power source is available.